1. Field of the Invention
This invention relates to post-cure processing of pneumatic tires in order to obtain optimum uniformity, and specifically to reductions of force variations in the tangential or running direction.
2. Description of the Prior Art
Due to non-uniformities in tire construction, it has been observed that extraneous forces and force variations are produced by the tire when the tire is rotating under load. Non-uniformities in the construction of the tire create moments and forces which can have an adverse effect upon tire ride and comfort.
Certain of these force variations are well known. The reduction of force variations in the radial direction is disclosed in U.S. Pat. No. 3,724,137. In radial force variation correction, the tire is mounted on a wheel rim, inflated to normal pressure, and rotated under a predetermined load against a loading drum. Radial force and radial force variation are measured on the loading drum by force transducers located in the radial direction on the axis of the loading drum. A pair of rotary grinders positioned adjacent the shoulder of the tire tread are moved into grinding engagement with the tread shoulder ribs in accordance with the radial force variations detected on the drum. These rotary grinders remove material from the shoulder ribs so that the tire becomes more uniform and the radial force variations are reduced to acceptable levels.
Due to the slowness of the response of the electromagnetic servo-system which controls the movement of the grinders, the tire is rotated at a fairly low speed. Typically, the tire is rotated at 60 rpm. This speed is sufficient to detect radial and lateral force variations since such variations exist independent of the speed at which the tire is rotated, in the absence of resonances.
It has been found that certain ride disturbances occur as a result of tire non-uniformities deriving from tangential or traction forces, or those forces parallel to the wheel plane in the direction of motion of the tire. These ride disturbances are evident with tires operating at all speeds. Automobiles equipped with radial or other types of tires have shown such ride disturbances at speeds between 0 and 80 mph, even with tires of minimal radial and lateral force variation characteristics. These ride disturbances take the form of a vehicle shake--generally a vibration felt once per wheel revolution being influenced by vehicle "wheel hop", and other vehicle resonances.
Tangential force variations or torque variations are speed dependent and may be generated by a change in angular acceleration of the tire which occurs over a portion of the circumference of the rotating tire. Speeds greater than 60 rpm and possibly in the order of 300 rpm or greater, equivalent to about 30 mph, are necessary for such tangential force variations to be accurately detected.
Current methods and apparatus for improving tire uniformity do not provide for reduction of tangential force variations but are rather concerned primarily with the reduction of radial force variations.